1. Field of the Invention
This invention relates to an analytical element for analytical chemistry, especially for analysis of the predetermined specific component in a fluid. More particularly, it is concerned with a quantitative analytical element for quantitative analysis of calcium in a biological fluid sample.
2. Description of the Prior Art
A number of methods for analysis of a component specimen in a fluid sample has been hitherto developed and roughly classified into two types, namely analysis reaction in a solvent system (hereinafter referred to as "wet chemistry") and analysis reaction in a solid phase system (hereinafter referred to as "dry chemistry").
The wet chemistry is known to include such analytical method as the so-called manual method without employing any device or apparatus up to the automatic quantitative analysis using a device or apparatus which have recently been employed in clinical test laboratories of hospitals.
As a determination method for calcium in a living body specimen according to the wet chemistry, there are mentioned a method with oxalate precipitation--potassium permanganate titration, the chelatometric titration method and the like.
The method with oxalate precipitation--potassium permanganate titration is to precipitate calcium as calcium oxalate, dissolve it in sulfuric acid and then determine the so-released oxalic acid according to calcium permanganate titration. However, this method has disadvantages of complicated procedures and inefficiency.
The chelatometric titration method is a method wherein calcium is converted to its chelate with ethylenediaminetetraacetic acid and the like and disapperance of calcium is determined from color change of an indicator to measure calcium concentration. However, this method requires complicated procedure in order to avoid influence of the co-existing magnesium and there is also a precision problem owing to the indirect determination of calcium concentration by back titration.
Generally speaking, the aforesaid methods have the disadvantages of complicated procedures, skilled operating techniques which are required and waste of time and labor.
On the other hand, the dry chemistry has been widely applied. For instance, the chemistry is conducted by impregnating a water absorbing carrier, e.g. a filter paper with an indicator solution followed by drying, as disclosed in U.S. Pat. Nos. 3,050,373, 3,061,523 and so on. Said method generally is to add a drop or drops of a fluid specimen to an analytical test paper or simply to a test strip or dip a test piece into a fluid specimen and to determine color change or density change in the test strip by visual observation or a reflection densitome, thereby measuring the concentration level of a specific component in a fluid sample.
These test methods are useful because of their simple procedures and results obtained in a short period of time, but they are not yet beyond the region of qualitative analysis, in view of a poor quantitativeness.
In contrast to the prior analytical methods as depicted above, there has been proposed an element for liquid analysis, as disclosed in U.S. Pat. No. 4,166,093, by the use of a conveniently operated dry chemistry with a high quantitative ability. This element is an analytical element which comprises a reagent layer placed over one side of an optically transmitting, liquid impermeable carrier or support and containing at least one reagent capable of reacting with a component in a fluid sample; a porous radiation blocking layer which transmits the said component in a fluid sample into the said reagent layer; and a radiation transmitting, detectable substance migration inhibiting layer interposed between the said two layers (U.S. Pat. No. 4,166,093).
According to the above-defined construction, calcium in a fluid sample can form a chelate with the indicator present in a reagent layer, for example, sodium 3,6-bis(4-chloro-2-phosphonophenylazo)-4,5-dihydroxy-2,7-naphthalenedisulf onate or 1,8-dihydroxynaphthalene-3,6-disulfonic acid-2,7-bis(azo-2-phenylarsonic acid) and the like with coloration. This method, unlike the wet chemistry, is convenient and quantitative generally to divalent metal ions, but has the great disadvantage of poor specificity to calcium with a remarkably lowered precision in calcium quantitative determination.
Moreover, the analytical element using 1,8-dihydroxynaphthalene-3,6-disulfonic acid-2,7-bis(azo-2-phenylarsonic acid) as disclosed in the above U.S. patent may show absorbance up to around 650 nm, while calcium concentration is determined upon absorbance at 680 nm produced when the said acid forms a chelate with calcium, and thus there is the disadvantage of a large error produced from some shifts of measurement wave length.